1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and more particularly to a developing device included in an image forming apparatus and capable of obviating erroneous toner content sensing when toner is not replenished.
2. Description of the Background Art
It is a common practice with an image forming apparatus to use a developer for developing a latent image formed on an image carrier, e.g., a photoconductive element. The developer is, in many cases, implemented as a toner and carrier mixture or two-ingredient type developer. Toner and carrier constituting this type of developer are agitated and mixed together in a developing device. As a result, the toner is charged to preselected polarity by friction. When the toner is consumed due to repeated development, fresh toner is replenished to the developing device to thereby maintain the toner content of the developer constant.
The toner content of the developer may be determined in terms of the permeability of the developer that varies in accordance with the bulk density of the carrier. The toner content of the developer is sensed not only when fresh toner is replenished, but also when it is not replenished with the developer being agitated. When fresh toner is not replenished, the toner content of the developer in the developing device is not expected to vary. In practice, however, when the amount of charge deposited on carrier grains by friction increases, the carrier grains repulse each other and reduce the bulk density of the developer. Consequently, the toner content is sensed in the same manner as when fresh toner is replenished, resulting in erroneous sensing of toner replenishment.
A sensor is used to sense the toner content of the developer being agitated. To protect the sensor from erroneous sensing, Japanese Patent Laid-Open Publication No. 5-150650, for example, discloses an arrangement including a cleaning blade mounted on a member that conveys a developer. The cleaning blade wipes the sensing surface of the sensor. In addition, fluorine is coated on the cleaning blade and the inside of a case around the sensing surface. This arrangement prevents the toner from depositing on the sensing surface.
Japanese Patent Laid-Open Publication No. 5-323794, for example, teaches a sensor protruding into the space of a developing device storing a developer through the wall of the developing device. An agitating member has a blade whose diameter is small so as not to interfere with the sensor protruding into the above space.
The prior art arrangements described above address the deposition of the developer on the sensing surface of the sensor, but do not address erroneous toner content sensing to occur when fresh toner is not replenished. More specifically, the prior art arrangements each agitate the developer when toner is not replenished, so that the bulk density of the developer is expected to vary. However, neither one of the above arrangements includes a measure against a decrease in the bulk density of the developer due to agitation to occur when toner is not replenished.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-19315 and 2000-56555.
It is an object of the present invention to provide a developing device capable of obviating erroneous toner content sensing when fresh toner is not replenished, and an image forming apparatus including the same.
A developing device of the present invention includes a conveying member for conveying a developer consisting of toner grains and carrier grains. A sensor senses the toner content of the developer in terms of the permeability of the developer. A preventing member prevents the bulk density of the developer from varying when fresh toner is not replenished.
An image forming apparatus including the developing device is also disclosed.